The renderer of KWin is one thing, but I think that Plasma (or better, the
QtQuick 2 that is used by Plasma) requires OpenGL by default (unless you have
the new 2D renderer with a recent version of Qt[1]).
Maybe Plasma did not start because the VM had no support for an OpenGL driver
(like LLVMPipe)?
[1] https://blog.qt.io/blog/2015/01/22/introducing-the-qt- quick-2d-renderer/